1. Field of the Invention
The invention concerns an imaging system and in particular a projection objective of a microlithographic projection exposure apparatus.
2. State of the Art
Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is known as a projection exposure apparatus comprising an illumination system and a projection objective. The image of a mask (=reticule) illuminated by means of the illumination system is in that case projected by means of the projection objective on to a substrate (for example a silicon wafer) which is coated with a photosensitive layer (photoresist) and which is arranged in the image plane of the projection objective, in order to transfer the mask structure on to the photosensitive coating of the substrate.
Particularly in the early days of microlithography, so-called ‘copy systems’ with an imaging scale β of close to 1 were used as the projection objectives. Designs by way of example of such ‘copy systems’ are set forth hereinafter without laying any claim to completeness and without a definitive appreciation thereof, as state of the art.
JP 2003185923 A discloses such a projection objective which is designed for a working wavelength of 365 nm and which has two catadioptric subsystems K1 and K3 with mutually displaced optical axes, between which an intermediate image is produced. Each of the subobjectives has a concave mirror for correction of image field curvature and a polarisation beam splitter for folding of the optical axis.
JP 2004086110 A discloses a purely refractive projection objective PL which is also designed for a working wavelength of 365 nm and between the lens groups G1 and G2 of which no intermediate image is produced and for which a numerical aperture of NA=0.275 is specified.
In order to do justice to the demands on manufacture of ever smaller structures in the μm range, present projection objectives for microlithography are typically designed in the form of reduction objectives with an imaging scale β of less than 1 and typically β=0.25 or less (for example with β=0.125, β=0.100 etc). That also reduces in particular the demands on the microstructuring of the mask (which depending on the above-mentioned imaging scale is 4 times, 8 times, 10 times etc larger). In order to correct the aberrations which occur in particular in such projection objectives which are of an ever more complex structure, high-resolution reduction objectives typically have a relatively large number of for example 20 or more optical elements such as lenses, mirrors, prisms and so forth.
Catadioptric designs which have both refractive and also reflecting components are particularly wide-spread. Admittedly, in such projection objectives, it is possible to achieve relatively high numerical apertures which can be further increased by means of immersion lithography to values above 1, but the process complication and expenditure in terms of production and adjustment is considerable in regard to the large number of different optical elements which are to be adjusted as precisely as possible relative to each other.
Furthermore, aspheric members are also used in the correction of aberrations, whereby further degrees of freedom are introduced in the optical system in particular without additional lens elements. In that respect it is known in particular that the position of aspherical surfaces in the system substantially influences the mode of action thereof on different kinds of aberration. An inventory of the ranges of action of aspheric members in dependence on the height of the outermost ray emanating from the object center beam and of the main ray is to be found for example in W Besenmatter: ‘Analyse der primären Wirkung asphärischer Flächen mit Hilfe des Delano-Diagramms’, [‘Analysis of the primary action of aspherical surfaces by means of the Delano diagram’], OPTIK, Vol 51, No 4, 1978, pages 385-396.